I can't really make this any more simplified than it's presented here:

"Most British drama or comedy shows have a very short season. The classic amount for comedies is just six episodes per series – The Office being a prime example – and there’s one very good reason for this. American comedy is a producer’s medium, in which an idea is worked up, characters developed and early scripts written, and then the show is handed over to a larger group of writers to flesh out into actual scripts. British comedy is a writer’s medium. The scripts are almost always written and developed by one or two people, then taken to production. Graham Linehan, the writer of Father Ted and The IT Crowd, even directs his own scripts, which is a LOT of work. And once they’ve written six episodes, they need a rest. That’s how we end up with only twelve episodes of Fawlty Towers (spread over TWO seasons, mind you) compared to the usual thrimpty-twelve of, say, Friends."

I've heard this as well, and I'm sure it contributes to most of it. That said, there's also the economics of it: it's a lot cheaper per episode to wring 22 episodes from a production company than 6 (diminishing costs, and all that); and the American audience is, of course, significantly larger than the UK's.